Domestication and breeding of tomatoes: what have we gained and what can we gain in the future?

Abstract

Background: It has been shown that a large variation is present and exploitable from wild Solanum species but most of it is still untapped. Considering the thousands of Solanum accessions in different gene banks and probably even more that are still untouched in the Andes, it is a challenge to exploit the diversity of tomato. What have we gained from tomato domestication and breeding and what can we gain in the future?

Scope: This review summarizes progress on tomato domestication and breeding and current efforts in tomato genome research. Also, it points out potential challenges in exploiting tomato biodiversity and depicts future perspectives in tomato breeding with the emerging knowledge from tomato-omics.

Conclusions: From first domestication to modern breeding, the tomato has been continually subjected to human selection for a wide array of applications in both science and commerce. Current efforts in tomato breeding are focused on discovering and exploiting genes for the most important traits in tomato germplasm. In the future, breeders will design cultivars by a process named 'breeding by design' based on the combination of science and technologies from the genomic era as well as their practical skills.

Fig. 1.

Collage depicting wide variation in size and shape of tomato fruit. (A) The large-fruited tomato ‘Giant Heirloom’ common to modern agriculture (right) and the typical fruit of a related wild species (L. pimpinellifolium). (B) The range of fruit size and shape variation in tomato. (C) Cross-section of the fruit from a plant homozygous for a mutation at the fasciated locus causing multiple locules. (D) Alternate allele of fasciated associated with unfused carpels. (E) Fruit from ‘Long John’, which carries mutations at both the sun and ovate loci causing extremely long, narrow fruit. (F) Bell pepper-type fruit produced by ‘Yellow Stuffer’. (G) Fruit from two different cultivars homozygous for a mutation at the ovate locus. In the variety on the left, the ovate mutation results in the production of fruit that are both elongated and constricted at the stem end of the fruit (hence, the pear shape). However, in the processing variety on the right, the ovate mutation causes elongated fruit but has a much reduced effect on neck constriction. This figure is from Tanksley SD. 2004. The Plant Cell 16: S181–S189.

Fig. 2.

Overview of mapped resistance genes and QTLs (quantitative trait loci) on the tomato genome. Marker loci are taken from the core FRLP map of Tanksley et al. (1992). Resistance genes and QTLs are printed in bold and italics with QTLs underlined. This figure is from Bai and Lindhout (2005).

Fig. 3.

Pyramiding introgression lines (ILs) into breeding lines to explore the genetic variation in Brix Yield (BY, measured in g m^–2). The upper panel shows the BY differences among different genotypes (shown as ▵ % from M82; base line represents M82) and the lower panel shows the combinations of tomato chromosomes 7, 8 and 9 in these genotypes. Yield differences between M82 (red chromosomes 7, 8 and 9), the four tomato tester inbreds (A = testers, pink chromosomes 7, 8 and 9), and their hybrids (B = M82 × testers) result from allelic variation present in the cultivated tomato gene pool. Since M82, the multiple-introgression line (IL789), and their hybrid IL789 × M82 (C = ILH789) differ only in three S. pennellii segments (green chromosomes), any BY difference between them is associated with the exotic allelic variation. The yield of the hybrids of IL789 with the four testers (D = IL789 × testers) results from both cultivated and exotic variation. This figure is from Gur and Zamir (2004) with modification.